It is known to prepare propylene oxide by the oxidation of aliphatic hydrocarbons such as propane or butane, or the oxidation of an olefin such as propylene.
In the Oxirane process, propylene oxide is prepared by the catalytic epoxidation of propylene with a hydroperoxide. In the process isobutane is oxidized in the liquid phase without catalyst at 120.degree. to 140.degree. C. and 30 to 35 atm. The resultant liquid is a mixture of reactant hydrocarbons, t-butyl hydroperoxide, t-butyl alcohol, acetone and lesser quantities of other by-products such as ketones, aldehydes, and acids.
Many of the by-products are carried over with the t-butyl hydroperoxide into the second reaction. In the second reaction, propylene is catalytically epoxidized with t-butyl hydroperoxide in liquid phase at high pressure to form propylene oxide. The preferred catalyst is a molybdenum naphthenate organic solution which has a 90 mol percent selectivity on propylene.
The reactions result in complex mixtures containing a number of oxidation products in addition to the propylene oxide. By-products include t-butyl alcohol, ethers, acids, glycols and esters such as methyl formate. Methyl formate has a boiling point near that of propylene oxide, making separation of the two by distillation impractical. In order to obtain propylene oxide suitable for applications, such as the manufacture of polyether polyols used in the preparation of polyurethanes, it is necessary to remove the methyl formate from the propylene oxide.
U.S. Pat. No. 3,477,919 teaches a method for purifying propylene oxide. Propylene oxide prepared by the oxidation of propylene is contaminated with impurities which boil near propylene oxide. The methyl formate impurity is removed from the contaminated propylene oxide by reaction with an aqueous slurry of calcium hydroxide.
U.S. Pat. No. 2,622,060 teaches a process for separating propylene oxide from a crude reaction mixture by treatment with an aqueous alkali metal hydroxide solution.
U.S. Pat. No. 2,550,847 teaches a process for the purification of propylene oxide in a crude reaction mixture containing methyl formate by subjecting the mixture to strong agitation with an aqueous solution of an alkaline saponifying agent.
U.S. Pat. No. 3,350,417 teaches a process for purifying propylene oxide comprising parallel and serial stages of distillation and a caustic treatment to simultaneously aldolize acetaldehyde and saponify methyl formate. The solvent used in the reaction step is removed before subsequent caustic treatment.
U.S. Pat. No. 3,838,020 teaches a process for purifying propylene oxide by extractive distillation with a dual solvent system.
All of these procedures are inherently inefficient in their removal of methyl formate. All of the procedures teach methods of formate removal which suffer from the disadvantage of hydrolysis of propylene oxide to propylene glycol resulting in a depletion of the desired propylene oxide product.
The instant invention is a practical and complete method of removing methyl formate accomplished without significant loss of propylene oxide.